1. Field of the Invention
The present invention relates to a vehicle engine. More particularly, the present invention relates to a variable compression ratio apparatus that changes compression ratio of an air-fuel mixture in a combustion chamber according to a driving condition of the engine.
2. Description of Related Art
Generally, thermal efficiency of combustion engines increases as the compression ratio thereof increases, and if ignition timing is advanced to some degree, thermal efficiency of spark-ignition engines increases.
However, if the ignition timing of the spark-ignition engines is advanced at a high compression ratio, abnormal combustion may occur and the engine may be damaged. Thus, the ignition timing cannot be excessively advanced and accordingly engine output may deteriorate.
A variable compression ratio (VCR) apparatus changes the compression ratio of an air-fuel mixture according to a driving condition of the engine.
The variable compression ratio apparatus raises the compression ratio of the air-fuel mixture at a low-load condition of the engine in order to improve fuel mileage. On the contrary, the variable compression ratio apparatus lowers the compression ratio of the air-fuel mixture at a high-load condition of the engine in order to prevent occurrence of knocking and improve engine output.
Currently, diesel engines achieve low-temperature combustion by enlarging the volume of a combustion chamber and by lowering the compression ratio in order to meet intensified exhaust gas regulations.
However, since startability at a cold temperature deteriorates as the compression ratio decreases, a glow plug system must be made of ceramic materials so as to strengthen them and an additional control unit for controlling the glow plug system is required. Thus, production costs may increase.
In addition, since the compression ratio is fixed, an optimal compression ratio according to a various driving conditions may not be achieved.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.